9 Random access single shared communication channel two or more simultaneous transmissions by nodes interference only one node can send successfully at a time multiple access protocol (Medium Access Control MAC): distributed algorithm that determines how stations share channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! can be synchronous or asynchronous

16 collisions can occur: propagation delay means two nodes may not year hear each other s transmission Collisions depend on the physical distance among nodes (the propagation delay) CSMA collisions spatial layout of nodes along ethernet

29 Ethernet Frame Structure Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported CRC: checked at receiver, if error is detected, the frame is simply dropped

30 Ethernet uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A}

31 Ethernet s CSMA/CD Jam Signal: make sure all other transmitters are aware of collision; Exponential Backoff: To adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3} after ten or more collisions, choose K from {0,1,2,3,4,,1023}

33 Ethernet limitation The proper functioning of Ethernet is guaranteed if a node is able to detect a collision. A collision has to be detected while transmitting a frame Condition: d trasm 2 d prop Given the network topology (the maximum distance among two hosts), it is possible to compute the minimum length of a frame Given the minimum length of a frame, it is possible to compute the maximum distance among two hosts or

34 (old) Ethernet devices Hub Physical Layer device Repeater operating at bit levels: repeat received bits on one interface to all other interfaces Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Bridge Link Layer device Operating on Ethernet frames, examining frame header and selectively forwarding frame based on its destination (MAC Forwarding table) Bridge isolates collision domains since it buffers frames When frame is to be forwarded, bridge uses CSMA/CD to transmit

Medium Access Protocols Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code Time Division,Code Division, Frequency Division Random partitioning

Chapter 5 Link Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can

Chapter 5 Link Layer and LANs A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you can add, modify, and

Data Link Protocols TCP/IP Suite and OSI Reference Model The TCP/IP protocol stack does not define the lower layers of a complete protocol stack In this lecture, we will address how the TCP/IP protocol

Chapter 6 The Link Layer and LANs A note on the use of these Powerpoint slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the

Chapter 6 The Link Layer and LANs A note on the use of these Powerpoint slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the

Chapter 5 Link Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can

Chapter 5 Link Layer and LANs A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you can add, modify, and

Lecture 4b Local Area Networks and Bridges Ethernet Invented by Boggs and Metcalf in the 1970 s at Xerox Local area networks were needed to connect computers, share files, etc. Thick or Thin Ethernet Cable

Chapter 5 Link Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can

The MAC Layer Jean Yves Le Boudec Fall 2012 1 Contents 1. MAC as Shared Medium : The Ethernet Myth and the WiFi Reality 2. MAC as interconnection at small scale : Why Ethernet became a point to point technology

Chapter 5 Link Layer and LANs A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you can add, modify, and

Chapter 6 The Link Layer and LNs note on the use of these Powerpoint slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations;

CSCI-1680 Link Layer Wrap-Up Rodrigo Fonseca Based partly on lecture notes by David Mazières, Phil Levis, John Jannotti Administrivia Homework I out later today, due next Thursday Today: Link Layer (cont.)

Local Area Networks LANs provide an efficient network solution : To support a large number of stations Over moderately high speed With relatively small bit errors Multiaccess Protocols Communication among

Distributed Queue Dual Bus IEEE 802.3 to 802.5 protocols are only suited for small LANs. They cannot be used for very large but non-wide area networks. IEEE 802.6 DQDB is designed for MANs It can cover